1. Field of the Invention
This invention relates to an automatic focussing device for objects to be recorded or reproduced such, for example, as a video disc.
2. Description of the Prior Art
In a video disc recording or reproduction apparatus, a recording or reading out luminous flux must be converged onto a video disc surface. In this case, provision must be made of an automatic focussing device which functions to bring an optical system for converging the above mentioned luminous flux onto the video disc surface in focus with the video disc surface.
Such kinds of automatic focussing devices have heretofore been proposed. In one device, a focussing luminous flux passes through an optical system and is reflected by a video disc surface and the configuration of the image reflected by the video disc surface is detected. The optical system is intentionally subjected to astigmatism and change of the configuration of the image reflected by the video disc surface in response to the position of the video disc surface is utilized to effect the focussing.
For example, an automatic focussing device for video disc in which is made of a cylindrical lens for the purpose of generating astigmatism has been proposed. In such conventional device, a luminous flux emitted from a light source passes through a half mirror and objective lens and is converged onto a video disc surface as a reading out beam of the luminous flux. A light reflected by the video disc surface passes through the cylindrical lens and is converged onto a detector. If the position of the video disc surface is changed, the above mentioned astigmatism of the cylindrical lens causes the configuration of the image produced on the detector to be changed. By detecting the change of such image configuration on the detector, it is possible to detect the change in position of the video disc surface and to always locate the video disc surface at a given position with respect to the optical system. Such conventional automatic focussing device has the disadvantage that the adjustment of the position of the detector is critical, and that the detection sensitivity is low.
In addition, another automatic focussing device in which astigmatism is produced on a luminous flux to be converged onto the video disc surface has also been proposed. Such conventional device, however, has the disadvantage that the astigmatism causes the light image converted onto the video disc surface to be enlarged, thereby rendering the reading out such enlarged light image difficult.